1. Field of the Invention
This invention relates generally to the field of electric motor controllers and, in particular, to optimizing the commutation delay of multipole brushless DC motors.
2. Background Art
The control and operation of multi-pole brushless DC motors that do not contain Hall effect position sensors requires position sensing via the back-EMF signals from the motor. Multi-pole motors are activated by sequentially energizing and de-energizing the windings. This process is commonly referred to as commutation. As the motor rotor moves, it generates voltages across the windings that are referred to as back-EMF (back electromotive force). The magnitude of the back-EMF signal from the motor winding not presently being activated is compared to a reference voltage in order to determine when to change the activation of the motor windings (perform commutation) to effect rotation. The point in time when the back-EMF crosses the reference is not the optimum time at which to commutate to the next coil activation state, but is earlier than the optimum time. Commutation at the optimum time is desirable because it maximizes motor torque and minimizes power consumption for a given rotational speed. When the motor is to be operated at a constant speed, optimum commutation can be achieved with a fixed time delay. However, if the motor is varying its speed (e.g., during acceleration), the amount of delay required is a variable. A method for generating the appropriate amount of variable delay is needed.
In the prior art, the period measurement and subsequent period division has sometimes been performed in a digital manner using counter/timers. This approach, however, requires significant circuitry to provide adequate time resolution.
Another method used in the prior art for period measurement and division involves analog circuitry which charges and discharges a multiplicity of capacitors. The analog method of the prior art requires several external components that are typically not included as part of an integrated circuit. These external components increase the complexity of the circuit and the amount of space that the circuit occupies on a circuit board. By increasing the complexity of the circuit, the reliability of the circuit is compromised.